This invention relates to machines designed to hold sheet metal blanks between upper and lower forming press tools. More specifically, this invention relates to devices designed to secure the sheet metal blank between the forming press tools in connection with superplastic, or other high temperature, forming.
Superplastic metallic alloys, such as certain alloys of aluminum, magnesium and titanium, are relatively ductile and can undergo substantial tensile deformation in the presence of low shaping forces. Sheet metal superplastic alloys can be shaped by a variety of processes and can be formed into the complex shapes characteristic, for example, of automotive body panels.
Forming presses have been used to manufacture vehicle body parts using superplastic aluminum alloys, such as very fine-grained AA5083. Often these presses comprise concave upper and convex lower tool members where a sheet metal blank is inserted and initially balanced on the convex lower member. The forming tools have been pre-heated and maintained at a suitable superplastic forming temperature, e.g., 800xc2x0 F., for the sheet metal blank. The blank itself may have been preheated to this temperature or will simply be heated in the forming press. And since the forming process is gradual and complex, the hot blank needs to be held in proper position between the forming tools.
As the upper concave tool member lowers and closes over the convex lower tool member during press operation, the sheet metal blank must be secured near its edges so that it is properly formed between the tools. Robots can be used to initially place the sheet metal blank on the lower tool in the hot environment. Presently, there exist blank holding devices that temporarily locate the edges or corners of the blank prior to upper tool closure. As disclosed in U.S. Pat. No. 6,085,571 entitled Gravity-Operated Blank Loading Device by Brinas et al., four loading devices, for example, are provided and used inside the press to position the four corners of a rectangular blank between the open tool members. However, these devices release the blank during tool closure. For this reason, there remains a need for a mechanism functional at press operating temperatures to hold the edges of the blank during tool closure for proper part manufacture. Unwanted movement of the blank leads to mal-formed parts and waste.
It is an object of the present invention to provide a blank loading device that is capable of positioning and securing the sheet metal blank to achieve proper alignment in the press that is maintained through out the entire press forming process.
The present invention provides a device used to position and securely hold a sheet metal blank in a sheet metal forming press operated at high temperatures. The forming press comprises an upper tool member having a concave surface and a lower tool member having a convex surface complementary to that of the upper tool member. The sheet metal blank is initially positioned in the forming press often by resting the sheet metal blank on the highest portion of the convex surface of the lower tool member. The sheet can be balanced, for example, by a gravity operated blank loading device much like that described in the ""571 patent. This invention provides a robust mechanism for gripping and securing near the edge of the blank so that it does not move out of position during tool closure.
In accordance with the present invention, sheet metal gripping mechanisms are suitably located at the margin(s) of the upper and lower forming tools. Generally, the forming tools are rectangular in outline and the subject gripping mechanisms would be attached to opposite sides of each tool. Where the inserted blank is balanced on the top of the lower convex tool, the subject gripping mechanisms are preferably located above and below these locations at the sides of the tools. The gripping mechanism carried on the upper tool is positioned to engage the margin of the sheet metal blank as soon as the tool is lowered from its open to its forming position. Preferably, this upper gripping mechanism comprises a block or beam with a knife-edge on its bottom surface. The edge is shaped to progressively engage and deform the sheet metal blank to grip it against a complementary gripping surface on the lower tool underlying the sheet.
As the upper tool lowers, the knife-edge engages the blank but the forming tool continues to drop. The upper knife-edged gripping mechanism must be capable of sliding up the side of the closing upper tool member in order to suitably maintain its grip on the edge of the sheet metal blank. Preferably, the gripping beam or block on the upper tool has sufficient weight or mass so that the knife-edge maintains a tight grip on the edge of the blank as it is undergoing substantial deformation between the working tools. In one embodiment, the gripping beam slides in slotted brackets mounted on the edge of the forming tool. The gripping mechanism comes into contact with the margin of the sheet metal blank and stops. The upper tool continues to lower and the gripping tool slides along its surface as it lowers. The weight of the gripping tool and the friction of the lowering upper tool contribute to the gripping force of the mechanism. Usually there will be at least two or even four such gripping devices attached to the ends of the upper tool member and each gripping device can include one or more knife-edges.
Underlying each of the upper tool gripping mechanisms is a complementary gripping surface located on the lower tool. In a preferred embodiment, each of these lower gripping surfaces remains stationary and are mounted so that they can bear the pressing force of the upper gripping tool and the blank. While the upper surface of the lower gripping device is generally flat like the blank margin that it is engaging, the device may contain a channel underlying the knife-edge of the upper mechanism so that sheet metal can be forced by the knife-edge into the channel to mechanically secure the blank. The channel also prevents the knife-edge from impacting the hard surface of the block, thus prolonging the life of the knife-edge blade. The sides of the channel in the lower gripping surface may be xe2x80x9cVxe2x80x9d shaped corresponding to the shape of the knife-edge in the upper gripping surface.
An important feature of the invention is the way in which the upper gripping mechanism is supported on the upper forming tool. As described above, it is necessary that the gripping mechanism be capable of sliding up the side of the forming tool as the forming tool closes. In one embodiment, the upper gripping mechanism is carried on a beam that spans the side of the upper tool and slides in slotted vertical brackets. In another embodiment, the upper knife-edged gripping device is carried on a block, which slides on two vertical rods fixed to the side of the upper tool. In still another embodiment, the upper gripping mechanism can be supported by both a rod and a slotted bracket. These features will be described in more detail below.